bash manual:

Bash performs the expansion by executing command in a subshell environment and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

## example 0

$a=$'a\nb\n\nc\n\n\n'
$echo -n "$a"
a
b

c

$echo -n "$(echo -n "$a")" a b c$